The sine-bar is used for angular measurements or for locating work at a given angle as, for example, in measuring or checking angles with accuracy using precision gage-blocks selected for whatever dimension is required in obtaining a given angle. Precision gage blocks are usually purchased in sets comprising a specific number of blocks of different sizes determined mathematically so that particular desired lengths can be obtained by combining select blocks, two or more blocks being combined by "wringing" them together with a swiveling motion causing them to adhere firmly one to another. When combining blocks for a given dimension, the object is to use as few blocks as possible, the procedure being based upon successive elimination of the right hand digit of the desired dimension. For example, the combined blocks measuring 0.1002+0.124+0.450+3.00 equals 3.6742 inches. An accurate surface plate or master flat is always used in conjunction with a sine-bar in order to form the base from which the vertical measurements are made, heretofore with precision gage blocks. A standard length for sine-bars is five inches (also ten inches) and there are tabulated constants that represent vertical heights H for setting a five inch sine-bar to the required angle. Assuming that the required angle is 31.degree. 20', the sine tables show that the height H should equal 2.6001 inches. Note that the constants in the sine tables equal five times the sine of angle; thus the sine of 31.degree. 20' in the trigonometric function table is 0.52002, and 0.52002.times.5 equals 2.6001 inches. There are many such functions for the use of a sine-bar, such as finding and checking angles, measuring angles and tapers and to determine center distances etc. However, note the complexity and precision of the involved process of using combinations of gage blocks according to the tabulated constants of the sine tables, in order to arrive at a precise angle; it being a general object of this invention to facilitate sine-bar operation with direct readings on the barrel of a micrometer that establishes the height H with precision and corresponding to the angular displacement of the sine-bar.
Sine-bars are precision devices for measuring angles accurately and locate work at a desired angle to some true surface, preferably a surface plate or the like. Sine-bars consist of a hardened, ground and laped steel bar which has accurately spaced cylindrical plugs of equal diameter attached to or near each end. The sine-bar has notched ends for receiving the cylindrical plugs which are held firmly against angular faces of the notch, for example exactly five inches or ten inches apart. The upper and lower sides of the sine-bars are parallel to the axes of the plugs within very close limits. The exact degree of angularity which a five inch or ten inch bar makes with a plane surface is obtained by determining the precise difference in vertical height of the spaced plugs. There are various forms of sine-bars and a refinement and/or carrying forward of the sine-bar concept involves simple and compound sine plates characterized by a precision plate pivoted by a plug at one end to a base and adapted to be elevated by the plug at the other end. In compound sine plates, the work supporting sine plate employes the underlying sine plate as its base. In any event, gage blocks have been used for elevating the sine plates respectively, and it is an object of the present invention to utilize the direct reading micrometer for elevation of the plug to be precisely positioned as to height H from the base plane or flat, all as hereinafter disclosed.
Micrometers take various forms, comprised generally of two visible elements and namely a barrel and a thimble, the barrel operating on a screw having forty threads per inch so that one revolution moves the barrel and thimble one fortieth of an inch. A feature is the relationship of calibrations on one element relative to an index line on the other element. In normal practice, the barrel is provided with a straight axially disposed index line stepped off incrementally in calibrations spaced 0.025 inch, and the thimble with an edge overlying said index line and calibrated circumferentially into twenty-five equal parts. Thus, each line on the thimble represents one thousandths of an inch, while each calibration line on the barrel represents twenty-five thousandths of an inch. It is an object of this invention to provide an inside micrometer that reads in degrees of angle as related to a sine-bar of given length, and wherein minutes of angle as well as seconds of angle are available as scaled readings. With the present invention, a vernier scale is provided in addition to the regular scales so that settings in degrees, minutes and seconds are available.